spawnProcess() passes incorrect environment to subprocess when env=None and posix_spawnp() is used

[p12tic]

Documentation on reactor.spawnProcess says the following about env parameter:

env is None: On POSIX: pass os.environ

However, twisted has this code in the code path leading to a call to posix_spawnp().

if environment is None:
    environment = {}

This leads to a subprocess being initialized with empty environment even though os.environ was expected.

Describe how to cause this behavior

There's a PR with automated tests added to Twisted.

Describe the correct behavior you'd like to see

Subprocess having parent process environment when invoked via reactor.spawnProcess(..., env=None).

Testing environment

• Operating System and Version;
  • Debian 12
• Twisted version: 23.10.0
• Reactor: default on Linux

Additional context
Probably a regression since 23.8.0 when posix_spawnp was enabled.